As computer frequencies and power increase, the radiated electromagnetic (EM) energy or noise increases proportionally. The EM energy originates from the different components in the computer system. This radiated noise interferes with the operation of other pieces of electronic equipment. Consequently, the radiating equipment is surrounded with a shield. Ideally, the best shield is a solid can, e.g. a box or sphere, that completely surrounds the radiating equipment. However, such a shield does not allow for air cooling of the equipment, as air flow would be blocked by the can. Thus, the equipment may overheat from lack of air flow.
One prior art solution is to use a perforated (perf) metal sheet as part of the EM enclosure. The perforations or holes in the enclosure would allow for air to enter and leave the system. Perf metal is made by punching an array of tightly spaced holes in a metallic sheet. When used as a shield for electromagnetic containment (EMC), perf metal allows for ventilation of an enclosure while offering some level of shielding from undesirable noise aft signals. In order for perf metal to provide effective shielding, each hole must be sized to a diameter that is small compared to the wavelength of the highest frequency to be shielded. In modem computer systems where shielding requirements typically extend to 5 Ghz or higher, hole diameters need to be very small. Holes as small as 2.0 mm are not uncommon. The following formula illustrates the relationship between attenuation (A in dB), frequency (f in 20 MHZ), hole diameter (d in M) and material thickness (t in M). ##EQU1##
This equation reveals that as diameter (d) increases, attenuation (A) decreases by the LOG base 10. Also, this equation shows that as thickness (t) increases, attenuation (A) increases linearly. Thus, hole diameter and sheet thickness are the two primary characteristics of the shield that affect attenuation. Note that the frequency (f) term in this equation is fixed by international regulatory requirements.
Since metal sheets are relatively thin, attenuation is derived mainly from having a small hole size. Thus, as the frequency of the EM noise increases (from increasing computer speeds) and the attenuation requirements increase, the required hole size is decreased. However, hole size can only be decreased to a certain point before a non-functioning structure results. For example, as hole size is decreased, air flow is diminished. This reduces the cooling effectiveness of the EMC sheet. Moreover, too small of a hole size may result in zero airflow from the effects of turbulence. Thus, manufacturability limits often preclude the use of perf metal entirely. For example, a practical limit for 25 dB of attenuation at 2 Ghz in a 1.25 mm thick panel, provides only 62% open area for ventilation. Increasing either airflow or attenuation requirements from these levels surpasses the useful domain of conventional perf metal.
Since air flow requirements drive hole size larger and EMC requirements drive hole size smaller, the remaining variables, i.e. metal thickness and boundary size, are pushed to the limit. However, minimum boundary distances between each hole must exist to prevent excessive metal deformation. In other words, if the holes are too close together, the metal sheet is sufficiently weakened to as to easily damage during manufacturing, installation or use. As indicated by the formula, the thickness of the sheet can be increased to increase the attenuation. However, this makes manufacturing of the sheets difficult in that the holes are gang punched and not drilled. Thus, as the sheet is made thicker, it becomes more difficult to punch holes without damaging the sheet. Therefore, the sheets must remain relatively thin. Consequently, it is difficult or impossible to construct a perf sheet vent panel capable of shielding to a pre-determined level up to regulatory frequency limits and still effectively vent heat. For example, a prior art perf metal sheet for 25 dB of attenuation at 2 Ghz in a 1.25 mm thick panel has holes that are 4 mm in diameter which provides only 62% open area for ventilation.
A prior art alternative to perf metal is honeycomb vent panel 30 as shown in FIG. 3. Honeycomb vent panel comprises a relatively thick structure with very small boundaries between holes 31. The small boundaries provide substantially increased open area for ventilation, while the thick structure greatly enhances shielding capability. Because of their thickness, each hole 31 acts as a waveguide (e.g. long conductive tube or transmission line), and thus attenuates the EM noise. The honeycomb panel is typically made out of foil sheets that are built on edge. Thin metal sheets 32 are stamped into corrugated shape, typically with three sides. Glue 33 is then applied to one of the three corrugated sides 34. The sheets are then aligned to form honeycombs, and secured in place via glue 33. The honeycomb panels are then cut to the desired thickness, typically one quarter to one half inch. The resulting structure is very light, and has 90% or greater open area for ventilation. Thus, the honeycomb vents will pass large amount of air.
However, honeycomb panels have some serious problems. They are expensive to construct, when compared with perf metal, because of the amount of processing involved. The panels are also difficult to mount because of the way the sheet elements are aligned and glued. The stresses on the sheets are large, and thus a strong glue is required. Note that the glues are non-conductors, and thus, the sheets do not have good electrical interconnections. This results in a degradation in performance. The interconnection forms a capacitor, and thus a good electrical connection between the sheets will not occur until very high frequencies are used. This problem can be remedied, but the remedy greatly increases the costs of the panel. For example, one possible remedy is to solder or weld the connection points instead of using an adhesive. If this remedy is not used, then the panel will perform inconsistently. Another problem with the panel is that the sheet construction makes the panels prone to dust clogging. The panel presents a sharp sheet edge in the airflow path. Lint and dust often has the form of a filament or a fiber. When the filament hits that edge, it tends to fall over it and become caught on the edge. Then other pieces of dust then are trapped by the filament, and stick to it. After awhile, the panel builds up a dust layer that completely clogs the holes. Thus, even though the holes are larger than perf metal, dust can clog the panel. Note that there are remedies for this problem, for example coating the panel with a material to round off the edges, but the remedies greatly increases costs of the panel, and thus are not typically used.
Another prior art EMC alternative is a mesh screen that is formed by interlaced metal wires. However, this alternative incurs problems similar to both the perf metal and the honeycomb panel. Note that the wires have a finite diameter, and as the holes approach the wire diameter, the open area is reduced to 50% or less. Also, like the honeycomb panel, dust tends to collect on the wires, and thus block airflow. Moreover, the wires do not have good electrical connections at their crossing junctions. Note that the wires are touching with a low contact force, and this does not form a good electrical connection. This type of connection results in only opportunistic contact between the wires. This can be remedied by soldering the crossing points, however, this is expensive and time consuming. Thus, performance is degraded. Therefore, mesh screens may not be suitable for EMC vent shields for the systems of today.
Therefore, there is a need in the art for an inexpensive vent panel that attaches to a device, and allows sufficient air to move through the panel to permit for heat transfer from the device, while attenuating EM noise originating within the device and not collecting dust.